Thank you for getting back to me so fast. I will be back at it again tomorrow. I will post what I find.

In my case was poor connection on control card...check those pins...connector housing wears out and pin moves inside too much creating all sorts of issues...good luck!!

I know this is an old post but was wondering if you ever learned what the issue was. I have a truck doing the exact same thing. I have found I can get it to happen consistently by traveling in one direction then while raising the platform, twist grip to plug. The same reaction happens. If I hold the grip to full throttle, BDI goes blank. Once throttle is released or I step off then right back on presence/brake pedal the BDI will re-illuminate and travel will be restored.

I tried swaping display modules with another truck but instead popped a code about wire guidance ( I dont remember the code)... I then tried another display from another truck and same code showed up , I put original display back into faulty truck and code dissapeared....I,m really having fun with these truck seriously lol..!!

sorry for taking so long to answer as I have being really busy (luckily enough on this bad economy)... ok I discarded to replace the transistor module and instead focused on simple stuff before moving deeper.

I checked all pins at control card for bad connections they checked good
I replaced key switch
I swapped traction motor
I replaced traction fuse
problem still persisting it does it at least 5 times in an 8 hour shift

liftdoctor has reminded me traction card also controls BDI
check wiring and voltages to card
99.99 pecent it wont be a motor or transistor fault so save your time
if you have a LX handset, plug into card and see if handset display goes blank when fault occurs
don't worry about that wriststrap, I haven't used it in years, as long as truck is powered down and your not wearing 1970's nylon jumpsuit

Thank you sir....yeah well the contactor was replaced and problem persisted..the battery swapped as well...currently has a new battery, only battery indicator goes blank...steering works smoth...I plan on swapping drive motor before transistor...I will look closely to that display module tought....currently buzy but will update...thanks!

This is an especially common problem with this truck. It is the forward/ reverse contactor. It give no codes. Replace the contactor, don't rebuild it. The contactor was warrantied for a year by Crown, but the tips only 90 days. The contactors can get weak coils and there was a little cushion washer that would bind up.. Crown blamed GE, and GE blamed Crown. If it an Albright contactor, replace it with the Albright for an RC3020-30. Crown will try to sell you a kit to replace the Albright with GE, but this was just a failed attempt to solve the problem. The GE contactor worked no better than the Albright. If it is GE, you must replace with GE. I replaced these all the time when working for Crown. This will work if everthing stays lit on the dashboard.

If the dashboard is going blank, you either have a battery problem or a commnication problem between the dashboard and the steering module. If a little green light is flashing a code 258 on the bottom of the dashboard, you have a communication problem. This is 2 flashes, 5 flashes, 8 flashes on the light. You also need to know that the transistor panel will not cause the display to go blank. I think you can unplug the panel, and the display will still work. It may show a code, but it will stay lit. The battery indicator may be blank, but your display will still work and show the hours, trouble codes, etc.

The communication problem is wiring or a bad steering or display module. If you are going to swap parts, swap your display or steering module with the other truck. I used to carry a spare display and a spare steering module from a scrap truck for the swap tronics style troubleshooting. Look at your plugs for your comm wires at the display, distribution panel and steering module. I think they are labeled comm+ and comm- on the wiring diagram.

Crownie has some good advice on the K-1 and the keyswitch. I would be looking at these. He also suggested checking the battery under a load. I would say anything under 17 or 18 volts when operating the truck is bad, and this is with a fully discharged battery. When charged ( and full of water), about 21 volts is about as low as voltage should go. This would be under full accceleration or full speed lift.

crownie: when truck faults no wrench light present...if I return grip to neutral bdi will not dissapear...currently working on other stuff and will post the result of transistor replacement after done... got to find my antistatic wrist band those mosfets are super sensitive on their gate!!

Thanks to both of you feels great to have support this forum is new to me, is a good idea to swap displays as is so easy..but...the fault happens before the display acts up....is only then when keeping a hold on the grip full speed that the bdi dissapears...looks like a pmt trip...guessing the transistor fails to turn off or on at high currents....by the way the mast cables were also replaced since this truck is from 1997 very old but we keep it mint..the parts came in...I will let you know...again thanks!!

yes, first rule of thumb
check the battery SOC first, load it up and see what voltage drop is.
I would also check the battery connector contacts, make sure they are not bent, one could be opening up breaking the circuit.
But what crownie says is the more likely scenario, a faulty relay or something.

try substituting the display panel and keyswitch
the bdi acting up points away from the traction panel
don't count out wiring as an intermittent fault could cause k relay to drop out
what is battery voltage like under load?
what happens if you don't hold accelerator grip?
are there any fault codes logged?